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Home My WebLink AboutRS-GEOT-2019-03-27-ESNWMarch 26, 2019 ES -3220.04 Mr. Don Sumpter P.O. Box 24943 Federal Way, Washington 98093 Subject: Groundwater Mounding Analysis Proposed Residential Development 14204 —156th Avenue Southeast Renton, Washington Earth I Solutions NW«C Earth Solutions NW LLC Geotechnical Engineering, Construction Obseivationjesting and Environmental Services Reference: Barghausen Consulting Engineers, Inc. Preliminary Grading and Drainage Plan, Sheet 4 of 7, dated December 13, 2018 Earth Solutions NW, LLC Geotechnical Engineering Study ES -3220.03, dated November 12, 2018 City of Renton, Washington Surface Water Design Manual Dated December 12, 2016 Dear Mr. Sumpter: In accordance with the request of Barghausen behalf, Earth Solutions NW, LLC (ESNW) has mounding analyses for the proposal. Pro'ect Description Consulting Engineers, Inc (BCE) acting on your prepared this letter detailing our groundwater The subject site is located east of the intersection between Southeast 146th Place and 156th Avenue Southeast, in Renton, Washington. We understand the current proposal includes construction of eight residential building lots, infrastructure improvements, and stormwater management facility. Current designs for the stormwater facility will consist of a R -Tank modular system to be located in the western project area designated Tract A on the referenced Drainage Plan. 1805 - 1361h Place N.E., Suite 201 • Bellevue, WA 98005 0 (425) 449-4704 0 FAX (425) 449-4711 Mr. Don Sumpter March 26, 2019 Subsurface Conditions ES -3220.04 Page 2 ESNW completed a geotechnical engineering study (study) for the subject project in November 2018. The study included excavation of six test pits within accessible areas of the site as well as two small scale pilot infiltration tests in September 2018. Native site soils primarily consist of sands and/or gravels, with varying silt contents, overlying sandy silt and silty gravel with sand. Based on the observed conditions, the sands appear to be thickest within the western site area. Groundwater seepage or the local groundwater table were not encountered during our September 2019 exploration. Groundwater Mounding Analysis We used the MODRET 6.1 hydrologic computer modeling software program to evaluate the predicted mounding of localized groundwater under the proposed facility during both the 100 - year peak rate event and cumulative highest volume event. The data set for both events were interpreted from values and records provided to us from BCE. The values were generated from the WWHM model. Various events of the same peak rate event were observed within the data set. The modeled maximum peak flow and volume events were used. A summary of the ModRet input parameters and predicted results are included in the attached ModRet summary. ModRet Parameters and Assumptions The data input included the proposed infiltration facility geometry, design high water, and bottom elevation and soil and groundwater conditions acquired from test pits completed within the project area. A local groundwater table was not encountered within the proposed stormwater facility area during our site exploration. As such, the deeper, hydraulically restrictive layer (glacial till) was used as the high groundwater (confining condition) elevation input for the model. A copy of the ModRet input parameters page is included in the attachment and a description of the parameters is included in Appendix A. A vertical hydraulic conductivity parameter was assigned from a representative infiltration test performed in the approximate location stormwater management facility. Horizontal hydraulic conductivity parameters were applied by ESNW based on the soil type encountered at the test pit location and the governing stormwater management manual guidance. Hydrograph The model hydrograph was generated from flow data provided to us by BCE using the WWHM model. The hydrograph data provided to ESNW were then converted to relevant units and input into MODRET as part of our analysis. Earth Solutions NK LLC Mr. Don Sumpter March 26, 2019 Results and Recommendations ES -3220.04 Page 3 We understand that an R -Tank located in the western property area will primarily manage stormwater runoff through infiltration. The facility has been designed with an overflow provision. The results of our analyses suggest that water will not mound into the system during the modeled 100 -year peak rate event and cumulative highest volume event. The model output suggests the lateral extent of the groundwater mound will decline relatively quickly as it moves away from the proposed facility towards the northerly and southerly property lines. Groundwater data was unavailable for neighboring properties; however, the model suggests the lateral mound may be approximately at elevation 367.5 feet near the south property line. This may result in saturation of local soils but is expected to be a transient condition. Topography shown on the referenced site plan indicates property line elevations to the north and south at about 374 feet in the area. As such, the lateral mound effects at the property line are projected to remain at least about six -and -one-half feet below surface elevations during and immediately after the modeled peak events. During normal rainy season conditions, we expect the effects of lateral mounding to be limited to elevated moisture content of adjacent soils. In our opinion, the results of the mounding analyses correlate with provided run-off data, facility geometry, and soil and groundwater conditions observed at the test pit locations. This opinion does not cover unforeseen or changed conditions. ESNW should observe the infiltration surface during construction to confirm soil conditions are as anticipated and to provide supplemental recommendations, if deemed necessary. City Review Comments Based on informal information provided by BCE, we understand a concern was raised by the City of Renton regarding observed caving within TP -1 and how that may potentially impact the proposed infiltration facility. In our opinion, the caving will not adversely impact the performance of the infiltration system or structural support. The low fines contents within the sands are susceptible to caving during excavation processes. This is not indicative of an unacceptable soil condition with respect to infiltration facilities. Earth Solutions NW. LLC Mr. Don Sumpter March 26, 2019 ES -3220.04 Page 4 We appreciate the opportunity to be of service to you and trust this letter meets your current needs. Should you have questions, or if additional information is required, please call. Sincerely, EARTH SOLUTIONS NW, LLC '0164PA I 49t Chase G. Halsen Senior Staff Geologist Inc R. CAMS of WAsit �� C� Kyle R. Campbell, P.E. Principal Engineer ot _e L. ., L. E. 4i�i-Pr sect Mana er Attachment: Appendix A — Mod Ret Design Parameters MODRET Computer Output cc: Barghausen Consulting Engineers, Inc. Attention: Ms. Ivana Halvorsen (Email only) Earth Solutions NW, LLC Appendix A ModRet Design Parameters ES -3220.04 Definition of Terms and Model Parameters This groundwater mounding analysis utilized ModRet V6.1 computer modeling software. The model was developed using a modified ModFlow program and uses site-specific parameters in the analysis. Groundwater mounding estimates use the Hantush equation developed in 1967 and is the standard of practice for estimating effects of groundwater mounding below infiltration facilities. Following are parameters used and the design rationale for each. Term Parameter Input Elevation of Effective Aquifer Base Measured top of the first low permeability stratum. This stratum was encountered at TP -1 at a depth of approximately 10 feet below the existing ground surface elevation (363.5 feet) Elevation of Starting Water Level Pond bottom elevation (371 feet) Average Effective Storage This parameter is a dimensionless value assigned by Coefficient soil type and groundwater/moisture conditions that approximates the volume of water that can be expelled or stored with a soil deposit. Using Table A-1 in the ModRet software program, a storage coefficient of 0.3 is assigned. Unsaturated Vertical Hydraulic Vertical infiltration rate based on in-situ infiltration Conductivity testing performed in vicinity to the stormwater facility converted to feet/day values. Saturated Horizontal Hydraulic This value is typically determined using the vertical Conductivity rate and applying a factor to that value. Published factors range from about 1 to over 10 depending on soil type. Our value is 1.5, which is conservative for this soil type. Earth Solutions NW. LLC MODRET SUMMARY OF UNSATURATED & SATURATED INPUT PARAMETERS PROJECT NAME: 3220.04 Sumptor Peak Volume MANUAL RUNOFF DATA USED UNSATURATED ANALYSIS EXCLUDED Pond Bottom Area 4,960.00 ft2 Pond Volume between Bottom & DHWL 3,470.00 ft3 Pond Length to Width Ratio (L/W) 1.52 Elevation of Effective Aquifer Base 363.50 ft Elevation of Seasonal High Groundwater Table 363.51 ft Elevation of Starting Water Level 371.00 ft Elevation of Pond Bottom 371.00 ft Design High Water Level Elevation 371.75 ft Avg. Effective Storage Coefficient of Soil for Unsaturated Analysis 0.30 Unsaturated Vertical Hydraulic Conductivity 50.00 ft/d Factor of Safety 2.00 Saturated Horizontal Hydraulic Conductivity 75.00 ft/d Avg. Effective Storage Coefficient of Soil for Saturated Analysis 0.27 Avg. Effective Storage Coefficient of Pond/Exfiltration Trench 1.00 Hydraulic Control Features: Top Bottom Left Right Groundwater Control Features - Y/N N N N N Distance to Edge of Pond 0.00 0.00 0.00 0.00 Elevation of Water Level 0.00 0.00 0.00 0.00 Impervious Barrier - Y/N N N N N Elevation of Barrier Bottom 0.00 0.00 0.00 0.00 Analysis Date: 03/26/2019 MODRET TIME - RUNOFF INPUT DATA r - PROJECT NAME: 3220.04 SUMPTOR PEAK VOLUME Analysis Date: 03/26/, STRESS FINCREMENT PERIOD OF TIME NUMBER (hrs) Unsat 0.00 1 24.00 VOLUME OF RUNOFF (ft3) 0.00 3.00 2 24.00 24.00 3,623.00 3 10,878.00 4 24.00 5,526.00 23,740.00 5 24.00 6 24.00 7,739.00 7 24.00 3,482.00 8 24.00 5,049.00 I 9 24.00 5,376.00 10 24.00 2,325.00 11 24.00 6,000.00 12 24.00 3,704.00 13 24.00 1,033.00 14 24.00 2,652.00 15 24.00 655.00 16 24.00 29.00 17 24.00 149.00 18 24.00 2,293.00 19 24.00 2,644.00 20 24.00 3,607.00 21 24.00 3,574.00 22 24.00 4,865.00 23 24.00 1,177.00 24 24.00 576.00 25 24.00 4,293.00 26 24.00 1,734.00 27 24.00 756.00 .019 28 24.00 1,159.00 29 24.00 1,480.00 MODRET SUMMARY OF RESULTS PROJECT NAME: 3220.04 Sumptor No Geo model CUMULATIVE TIME (hrs) WATER ELEVATION (feet) INSTANTANEOUS INFILTRATION RATE (cfs) AVERAGE INFILTRATION RATE (ds) CUMULATIVE OVERFLOW (ft3) 00.00-0.00 363.510 0.000 * 0.00000 0.00 363.510 0.17023 0.13546 24.00 371.000 0.10068 0.00 0.06591 48.00 371.000 0.07619 0.00 0.08648 72.00 371.000 0.07649 0.06650 0.00 96.00 371.000 0.11282 0.00 0.15913 120.00 371.000 0.13017 0.00 0.10122 144.00 371.000 0.08356 0.00 0.06549 0.06590 0.06508 0.00 168.00 371.000 192.00 371.000 0.06563 0.06617 0.00 216.00 371.000 0.05786 0.04955 0.00 240.00 371.000 0.05596 0.00 0.06238 0.00 264.00 371.000 0.05750 I 0.05262 0.00 288.00 371.000 0.04373 0.03484 0.00 312.00 371.000 0.03534 Analysis Date: 03/21/2019 MODRET SUMMARY OF RESULTS PROJECT NAME: 3220.04 Sumptor No Geo model CUMULATIVE TIME (hrs) WATER ELEVATION (feet) INSTANTANEOUS INFILTRATION I RATE (ds) AVERAGE CUMULATIVE INFILTRATION OVERFLOW RATE (cfs) (ft3) 0.03584 336.00 371.000 0.03019 0.00 0.02454 360.00 371.000 0.02057 0.00 0.01660 384.00 371.000 0.01486 0.00 0.01311 408.00 371.000 0.01700 0.00 0.02088 432.00 371.000 0.02274 0.00 0.02460 456.00 371.000 0.02765 0.00 0.03069 480.00 371.000 0.03193 0.00 0.03317 504.00 371.000 0.03709 0.03327 0.00 0.04101 0.00 0.02554 528.00 371.000 552.00 371.000 0.02163 0.00 0.01771 0.00 576.00 371.000 0.02521 0.02865 0.03270 600.00 371.000 0.00 0.02460 0.00 0.01742 624.00 371.000 0.02101 648.00 371.000 0.01707 1 0.00 Analysis Date: 03/21/2019 MODRET SUMMARY OF RESULTS PROJECT NAME: 3220.04 Sumptor No Geo model CUMULATIVE WATER INSTANTANEOUS TIME ELEVATION INFILTRATION (hrs) (feet) RATE (cfs) AVERAGE INFILTRATION RATE (cfs) CUMULATIVE OVERFLOW (ft3) 0.01672 672.00 371.000 0.01716 0.00 0.01760 696.00 368.583 0.00 Maximum Water Elevation: 371.000 feet @ 24.00 hours * Time increment when there is no runoff Maximum Infiltration Rate: 2.972 ft/day Analysis Date: 03/21/2019 Recovery @ 148.494 hours MODRET SUMMARY OF UNSATURATED & SATURATED INPUT PARAMETERS PRO]ECT NAME: 3220.04 Sumptor Peak Flow MANUAL RUNOFF DATA USED UNSATURATED ANALYSIS EXCLUDED Pond Bottom Area Pond Volume between Bottom & DHWL Pond Length to Width Ratio (L/W) Elevation of Effective Aquifer Base Elevation of Seasonal High Groundwater Table Elevation of Starting Water Level Elevation of Pond Bottom Design High Water Level Elevation Avg. Effective Storage Coefficient of Soil for Unsaturated Analysis Unsaturated Vertical Hydraulic Conductivity Factor of Safety Saturated Horizontal Hydraulic Conductivity Avg. Effective Storage Coefficient of Soil for Saturated Analysis Avg. Effective Storage Coefficient of Pond/Exfiltration Trench Hydraulic Control Features: Top Groundwater Control Features - Y/N N Distance to Edge of Pond 0.00 Elevation of Water Level 0.00 Impervious Barrier - Y/N Elevation of Barrier Bottom Analysis Date: 03/26/2019 4,960.00 ftz 3,470.00 ft3 1.52 363.50 ft 363.51 ft 371.00 ft 371.00 ft 371.75 ft 0.30 50.00 ft/d 2.00 75.00 ft/d 0.27 1.00 Bottom Left Right N N N 0.00 0.00 0.00 0.00 0.00 0.00 N N N N 0.00 0.00 0.00 M Analysis Date: 03/26/ MODRET TIME - RUNOFF INPUT DATA PROJECT NAME: 3220.04 SUMPTOR PEAK FLOW STRESS PERIOD NUMBER INCREMENT OF TIME (hrs) VOLUME OF RUNOFF (ft3) 0.00 224.00 Unsat 0.00 1 24.00 2 24.00 355.00 3 24.00 66.00 7,386.00 4 24.00 5 24.00 3,359.00 6 24.00 880.00 7 24.00 437.00 8 24.00 21.00 9 24.00 109.00 54.00 10 24.00 11 24.00 1,052.00 12 24.00 1,520.00 3,549.00 13 24.00 14 24.00 2,494.00 15 24.00 24.00 1,765.00 16 1,141.00 17 24.00 268.00 18 24.00 132.00 19 24.00 2,061.00 20 24.00 6,796.00 21 24.00 5,336.00 22 24.00 3,221.00 23 24.00 1,530.00 24 24.00 1,860.00 25 24.00 24.00 24.00 18,004.00 26 9,026.00 27 3,180.00 1019 28 24.00 1,443.00 29 24.00 2,609.00 CUMULATIVE TIME (hrs) 00.00-0.00 0.00 24.00 48.00 72.00 96.00 120.00 144.00 168.00 192.00 216.00 240.00 264.00 288.00 312.00 Analysis Date: 03/26/2019 MODRET SUMMARY OF RESULTS PROJECT NAME: 3220.04 Sumptor Peak Flow WATER ELEVATION (feet) 363.510 363.510 371.000 371.000 371.000 371.000 371.000 371.000 371.000 371.000 371.000 371.000 371.000 371.000 371.000 INSTANTANEOUS AVERAGE INFILTRATION INFILTRATION RATE (ds) RATE (cfs) 0.000 * 0.17970 0.09357 0.03943 0.04007 0.04619 0.03303 0.02142 0.01449 0.01048 0.00866 0.00973 0.01314 0.01986 0.02495 0.00000 0.13664 0.05051 0.02835 0.05179 0.04059 0.02547 0.01737 0.01161 0.00934 0.00797 0.01149 0.01479 0.02494 CUMULATIVE OVERFLOW (ft3) 161011]:8 A I SUMMARY OF RESULTS PROJECT NAME: 3220.04 Sumptor Peak Flow i CUMULATIVE WATER INSTANTANEOUS AVERAGE TIME ELEVATION INFILTRATION INFILTRATION (hrs) (feet) RATE (cfs) RATE (ds) 7- CUMULATIVE OVERFLOW (ft3) 0.02496 336.00 371.000 0.02367 0.00 0.02238 360.00 371.000 0.02062 0.00 0.01885 384.00 371.000 0.01624 0.00 0.01363 408.00 371.000 0.01215 0.00 0.01067 432.00 371.000 0.01376 0.00 0.01686 456.00 371.000 0.02833 0.00 0.03980 480.00 371.000 0.04164 0.00 0.04348 504.00 371.000 0.04012 0.00 0.03677 528.00 371.000 0.03196 0.00 0.02714 552.00 371.000 0.02612 0.00 0.02509 576.00 371.000 0.06499 0.00 0.10488 600.00 371.000 0.09727 0.00 0.08966 624.00 371.000 0.07374 0.00 0.05782 648.00 371.000 0.04842 0.00 -41 Analysis Date: 03/26/2019 CUMULATIVE TIME (hrs) MODRET SUMMARY OF RESULTS EPROJECT NAME: 3220.04 Sumptor Peak Flow WATER INSTANTANEOUS AVERAGE ELEVATION INFILTRATION INFILTRATION (feet) RATE (ds) RATE (ds) r111xl1N CUMULATIVE OVERFLOW (ft3) 1 672.00 1 371.000 1 0.03801 1 1 0.00 0.03700 1 696.00 1 368.864 1 1 1 0.00 1 Maximum Water Elevation: 371.000 feet @ 24.00 hours * Time increment when there is no runoff Maximum Infiltration Rate: 2.552 ft/day Analysis Date: 03/26/2019 Recovery @ 0.000 hours